Characteristics of an ink having a shear thinning property have so far been utilized in gel ink ballpoint pens. The above ink composition for a ballpoint pen contains a thickener (gelling agent) showing a shear thinning property, and known are, for example, 1) an ink composition for an aqueous ballpoint pen characterized by containing 0.20 to 0.45% by weight of a xanthan gum (refer to, for example, a patent document 1) and 2) an aqueous ink composition for a writing instrument suitable for a ballpoint pen, comprising as essential components, a) a colorant, b) an organic acid-modified heteropolysaccharide which comprises a fundamental unit constituted by glucose/galactose/pyruvic acid or a salt thereof/succinic acid or a salt thereof/acetic acid of 5 to 8/1 to 2/0.5 to 2/0.5 to 2/0.5 to 1 in terms of a molar ratio and which has an average molecular weight of about 1,000,000 to about 8,000,000 and c) an aqueous medium comprising water and a water-soluble organic medium, wherein water accounts for 50% by weight or more (refer to, for example, a patent document 2).
The above gel inks having a shear thinning property are characterized by that they have, as shown in FIG. 1, a high viscosity when not used for writing (in non-writing) and are inhibited from dropping and that when used for writing (in high shearing), they are decreased in a viscosity by virtue of an action of a rotating ball to make it possible to write smoothly. They show a viscosity flow curve in which a viscosity is gradually reduced as a shear rate is elevated.
However, the conventional gel inks having such the viscosity activities as described above have the problem that they have a heavy writing feeling at an initial part of writing and are liable to be blurred and drop. Further, involved therein is the problem that the inks are extremely reduced in a viscosity during writing, whereby the drawn lines are decreased in a density in a stop part and split in a sweep part, and the sharp drawn lines tend to be less likely to be obtained.
On the one hand, for example, an aqueous pigment ink comprising at least titanium oxide, an extender pigment, a dispersant, an emulsion of polyvinyl alcohol and/or a styrene•acryl base copolymer each having a polymerization degree of 200 to 1000, and water (refer to, for example, a patent document 3) is known as an ink for a writing instrument such as a marking pen and a ballpoint pen, in which polyvinyl alcohol is used.
On the other hand, an aqueous ink composition comprising alkali metal silicate and/or alkali metal borate in order to improve a corrosion resistance of a cemented carbide-made ball at a tip part of the pen, and an aqueous ballpoint pen charged with the same (refer to, for example, a patent document 4) are known as an aqueous ink composition in which alkali metal borate is used.
However, the polyvinyl alcohol in the ink for a writing instrument described in the foregoing patent document 3 is used in order to improve a dispersion stability of the pigment in freezing and thawing, and the objects and the problems are different from those in the present invention. In addition thereto, an ink having a dilatant fluid characteristic is neither described nor recognized therein at all.
Further, the alkali metal borate in the aqueous ink composition described in the patent document 4 described above is used in order to form a thin film of the above alkali metal borate on the ball to prevent the ball and the tip holder from being corroded, and the objects and the problems are different from those in the present invention. In addition thereto, an ink having a dilatant fluid characteristic is neither described nor recognized therein at all.